1. Field of the Invention
The present invention relates to a controlling method for an electronic still camera which picks up an image signal through a solid state imaging device such as a CCD (charge coupled device) image sensor, and records still pictures on a signal recording medium such as a card memory or a magnetic disc. More particularly, the present invention relates to a recording control method for an electronic still camera with an electronic view finder for displaying a moving picture of a photographic subject.
2. Background Arts
Electronic still cameras have been known, wherein a solid state imaging device photo-electrically converts an optical image into electric image signals, and the image signals are recorded as a still picture on a signal recording medium. The recorded image signal is read to display the still picture on a screen such as an LCD (liquid crystal display) or a CRT (cathod ray tube). Some still cameras adopt an optical view finder. But providing an optical view finder raises the cost of the still camera. In addition, the optical view finder is incapable of forming a clear image of a low brightness subject. Therefore, many still cameras adopt an electronic view finder to display a moving picture of photographic subject on a small LCD or a small CRT on the basis of image signals picked up through the same solid state imaging device that is used for recording, as disclosed for example in JPA 4-348685.
A solid state imaging device has a matrix of photo-electric sensor cells or pixels on its imaging surface, each pixel is cyclically charged with a photo-electric signal of an amount corresponding to received light quantity, and the charge stored in the pixel is read as an image signal. In front of the pixels, there are blue, green and red filters, so each pixel receives one of three colors of light. The charges are transferred in series to a signal processor, to be processed as three color image signals.
For the electronic view finder, image signals obtained through the solid state imaging device are transformed into an appropriate form for displaying a moving picture on the LCD or CRT. For example, to obtain field image signals according to the NTSC system, the solid state imaging device is driven at intervals of 1/60 seconds corresponding to the field frequency 60 Hz of the NTSC system, to sample signal charges of the pixels in accordance with the interlace-scanning method. By interlacing a field with a next field, a frame is displayed every 1/30 seconds on the LCD of the electronic view finder or on the CRT. Thus, a moving picture of a subject that the still camera is photographing is displayed on the electronic view finder in a real time fashion. Besides the function of framing, the electronic view finder allows to monitor the recorded still picture on the instant based on the image signals read from the signal recording medium.
As a method of interlacing the pixels of the solid state imaging device, recent video cameras adopt a charge transfer method, hereinafter referred to as pixel combination. According to the pixel combination method, an image signal of one color is obtained as a sum of signal charges of two adjacent pixels which receive the same color. Specifically, image signals for a first or odd field are obtained by adding the signal charge of each of those pixels aligned in even horizontal scanning lines of the CCD image sensor to the signal charge of one of those pixels detecting the same color in one of two adjacent odd horizontal scanning lines. Thereafter, to provide image signals for a second or even field, the signal charge of each pixel of the even horizontal scanning lines is added to the signal charge of one of those pixels detecting the same color in the other of the two adjacent odd horizontal scanning lines.
The field image signals, for the first and second fields are respectively processed for white balance correction and γ-correction in a conventional manner, and are interlaced with each other to display a frame of the moving picture on the electronic view finder. On the basis of the field image signals obtained in this way, the moving picture can also be displayed on an external monitor such as an external CRT with no problem.
On the other hand, when recording a still picture on the signal recording medium, it is desirable to read the signal charges of the individual pixels without combining or adding them, and record the image signals in the form of digital image data for each pixel, because the recorded image data of the still picture is expected to be processed for each pixel, e.g. in a personal computer to print or merge-print the still picture as a hard copy. Therefore, pixel-combined field image signals as above are inappropriate for recording. For this reason, the reading format of the solid state imaging device is switched over between when to pick up field image signals for displaying a moving picture, hereinafter called “a movie mode”, and when to pick up image signals for recording a still picture, hereinafter called “a recording mode”. In the recording mode, the still camera with the electronic view finder scans the individual pixels sequentially, and records an image signal for each pixel in the signal recording medium.
As a result, under the same exposure condition, the signal levels of the image signals for the still picture drops from those of the pixel-combined field image signals as being obtained by adding the signal charges of every two pixels. In the movie mode, white balance and exposure amount, e.g. charge storage time of the solid state imaging device, are automatically controlled depending upon the signal levels of the pixel-combined image signals. If the same white balance and the same exposure value as used for the movie mode are applied to the recording mode, reproduced still pictures would have improper low density and insufficient color balance due to the difference in signal level. However, controlling exposure based on the signal levels of image signals obtained from the individual pixels in the recording mode, i.e. after each shutter release, results in a longer time lag from the shutter release to the actual recording, so that the recorded still picture will be shifted in time from a desired one. This is apparently undesirable.